This disclosure relates to a noise removing apparatus and a noise removing method, and more particularly to a noise removing apparatus and a noise removing method which remove noise by emphasis of object sound and a post filtering process.
It is supposed that a user sometimes uses a noise canceling headphone to enjoy music reproduced, for example, by a portable telephone set, a personal computer or a like apparatus. If, in this situation, a telephone call, a chat call or the like is received, then it is very cumbersome to the user to prepare a microphone every time and then start conversation. It is desirable to the user to start conversation handsfree without preparing a microphone.
A microphone for noise cancellation is installed at a portion of a noise canceling headphone corresponding to an ear, and it is a possible idea to utilize the microphone to carry out conversation. The user can thereby implement conversation while wearing the headphone thereon. In this instance, ambient noise gives rise to a problem, and therefore, it is demanded to transmit only voice with noise suppressed.
A technique for removing noise by emphasis of object sound and a post filtering process is disclosed, for example, in Japanese Patent Laid-Open No. 2009-49998 (hereinafter referred to as Patent Document 1). FIG. 31 shows an example of a configuration of the noise removing apparatus disclosed in Patent Document 1. Referring to FIG. 31, the noise removing apparatus includes a beam former section (11) which emphasizes voice and a blocking matrix section (12) which emphasizes noise. Since noise is not fully canceled by the emphasis of voice, the noise emphasized by the blocking matrix section (12) is used by noise reduction means (13) to reduce noise components.
Further, in the noise removing apparatus, remaining noise is removed by post filtering means (14). In this instance, although outputs of the noise reduction means (13) and processing means (15) are used, a spectrum error is caused by a characteristic of the filter. Therefore, correction is carried out by an adaptation section (16).
In this instance, the correction is carried out such that, within an interval within which no object sound exists but only noise exists, an output S1 of the noise reduction means (13) and an output S2 of the adaptation section (16) become equal to each other. This is represented by the following expression (1):E{Ãn(ejΩμ,k)}=E{|A(ejΩμ,k)|2As(ejΩμ,k)=0}  (1)
where the left side represents an expected value of the output S2 of the adaptation section (16) while the right side represents an expected value of the output S1 of the noise reduction means (13) within an interval within which no object sound exists.
By such correction, within an interval within which only noise exists, no error appears between the outputs S1 and S2 and the post filtering means (14) can remove the noise fully, but within an interval within which both of voice and noise exist, the post filtering means (14) can remove only the noise components while leaving the voice.
It can be interpreted that this correction corrects the directional characteristic of the filter. FIG. 32A illustrates an example of the directional characteristic of a filter before correction, and FIG. 32B illustrates an example of the directional characteristic of the filter after correction. In FIGS. 32A and 32B, the axis of ordinate indicates the gain, and the gain increases upwardly.
In FIG. 32A, a solid line curve a indicates a directional characteristic of emphasizing object sound produced by the beam former section (11). By this directional characteristic, object sound on the front is emphasized while the gain of sound coming from any other direction is lowered. Further, in FIG. 32A, a broken line curve b indicates a directional characteristic produced by the blocking matrix section (12). By this directional characteristic, the gain in the direction of object sound is lowered and noise is estimated.
Before correction, an error in gain exists in the direction of noise between the directional characteristic for object sound emphasis indicated by the solid line curve a and the directional characteristic for noise estimation indicated by the broken line curve b. Therefore, when the noise estimation signal is subtracted from the object sound estimation signal by the post filtering means (14), insufficient cancellation or excessive cancellation of noise occurs.
Meanwhile, in FIG. 32B, a solid line curve a′ represents a directional characteristic for object sound emphasis after the correction. Further, in FIG. 32B, a broken line curve b′ represents a directional characteristic for noise estimation after the correction. The gains in the direction of noise in the directional characteristic for object sound emphasis and the directional characteristic for noise estimation are adjusted to each other with a correction coefficient. Consequently, when the noise estimation signal is subtracted from the object sound estimation signal by the post filtering means (14), insufficient cancellation or excessive cancellation of noise is reduced.